[swarming] add grace period option to swarming trigger command
[swarming] add grace period option to swarming trigger command

This is for http://cl/446996499

Change-Id: I3354b9155f07cec0b49f61fb92ebafc667dda62f
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3631684
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Auto-Submit: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Reviewed-by: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go